Two years ago, EAN reviewed the Pass Labs INT-150, a 150-watt Class-A/B MOSFET output version of the Nelson Pass-designed, super-symmetry amplifier — combined with its new preamp designs. We had lots of good things to say about this combo that, in many systems, are as good as or better than numerous audiophile separates.

The Pass Labs INT-Series was initially introduced for customers in the Asian market who often have a preference for high-end sound, in smaller packages. The demand for the Pass integrated series has since increased elsewhere in the world including the U.S, where it has become a best seller for the California company.

Pass has, hence forth, released an all Class-A version of the integrated series, the $7,150 INT-30A. This made-in-USA integrated sports the same passive preamp, features and, connectivity of the INT-150, but is biased in the delicious sounding, pure Class-A mode of the separate Pass amp, the XA-30.5.

Features

The INT-30A’s amp section is the the same Class-A super symmetry MOSFET design from the popular XA30.5 that is also used in the INT-150, but biased into Class A; the INT-150 makes 150 watts in Class A/B. This design utilizes carefully matched MOSFET output devices in a low-feedback, fewer-stage configuration. The circuit contains ultra-linear JFET input devices with high-bias Class A on the input stage with a limited feedback loop. The power supply sections is optimized for pure Class-A operation. with optimal filtering and more power supply capacitance. These design features allow this beauty to offer the exquisite immersive sound reproduction of Class A, with a tinge of warmness, yet with the impressive bass and transient speed.

The preamp side of the equation is Pass’s passive attenuation control — with multiple resistors — that utilizes the gain of the sources component to drive the amplifier. With most line-level sources, the INT-30A can generate plenty of level through its internal amplifier, to fill most rooms with that glorious Class-A sound that is quite accurate and dimensionaly detailed.

The more simple, nearly direct signal path of passive attenuation really complements the Class A amp, allowing the transients of a well recorded drum cymbals to jump out of the speakers with out any muted of the dynamics. It is smart way to do Class A amplification with an integrated preamplifier.

Yet, the more simple, nearly direct signal path of passive attenuation really complements the Class-A amp, allowing the transients of a well-recorded drum cymbals to jump out of the speakers without any muting of the dynamics. It is smart way to do Class-A amplification with an integrated preamplifier.

Wayne Colburn, a design engineer with Pass Labs, designed the preamp section in the INT series. “The integrated amplifiers mix a bit of our preamps and of course power amps,” he explained. “The input selection is done via low-level signal relays and is routed into a passive attenuator with 63 single dB steps.”

“For best performance this circuitry needs to be fed to a high impedance," he added. “This circuit is a Class-A, self-biased, unity gain buffer with no negative feed back. This output path is routed to the preamplifier output, as well as feeding the power amplifier section. In essence, Colburn noted, “the INT series use a combination of active and passive circuitry, or a minimalist preamplifier if you will, coupled to one of our standard power amps.”

INT-30A Rear Panel Configuration

Like its Class-A/B sibling, the INT-150, the INT-30A can accommodate most audiophile playback components. There are four stereo pairs of RCA single-ended inputs and two sets of balanced XLR inputs. (Inputs 1 and 2 are RCA or XLR). For those that want to use only the preamp signal section, the INT-30A has separate preamp outputs — RCA or balanced XLR, though only one or the other is advised to be connected at a time.

As is typical of Pass’ top-tier craftsmanship in audiophile components, the INT-30A’s features the same chassis as the INT-150. The front panel features a brushed silver faceplate, blue backlit status display, blue LED input indicators, separate push-button input and mute selectors, and a push-button power switch. The volume control has a fantastic, solid, tactile feel that oozes quality. One turn of the knob and you know the sound will be magnificent.

Although I am a traditional audiophile who likes the personal jaunts to the preamp controls, Pass has kindly installed a remote control circuit with a heavy-duty remote for those who like to sit in the chair and fine tune the preamp; the remote duplicates all the front panel functions, including source switching and mute.

As you would expect from Pass Labs, the INT-30A has great specs. It outputs 30 wpc at 8 ohms and 60 watts at 4 ohms. It crosses into Class A/B territory at above 60 watts. Distortion is under 1 percent at rated power and .02 percent at 5 watts — with a bandwidth from 1.5 Hz to 60 kHz, (-2 dB). The damping factor is spec’d at 150 ref. 8 ohms nominal, and the slew rate, +/- 50V/uS. The THD is listed at .003 percent at 5 watts output. The INT-30A’s specs show that it is no tweaky Class-A amp, but an amp that can handle many kinds of music and components in order to showcase your listening preference — from vinyl to high-resolution PCM and DSD.

The INT-30A’s physical dimensions and overall appearance are exactly the same as the INT-150. It weighs a stout 60 pounds and is 19-inches wide, 7-inches tall and 19-inches from front to back. The amp also sports those huge, side-mounted heat sinks to handle the Class-A operation. Power consumption is 225 watts at idle and 600 watts at full power. Gulp!

The setup

I installed the INT-30A in my main audiophile system, which always includes a variety of speakers, my reference multi-driver tower reference Legacy Focus20/20s, Legacy Studio HD and WestlakeLC 8.1 closefield monitors, as well as a little a new set of Westlake LC 2.65 midfield three-ways, and a brief run with the Volti Audio’s amazing single-driver Veretta. Sources included the Esoteric DV-50 and Oppo BDP-95 universal players and Benchmark Dac1 Pre24-bit D/A, and a Lavry DA10 D/A. I used Alpha-Core solid-silver interconnects and speaker cables. The INT-30A and all source components were plugged into an Essential Sound Products power strip with ESP Essence power cords.

**I also threw in my Rogue Audio Model 99 Magnum tube preamplifier and a Coda High Current bipolar output preamplifier. I also had on hand a number of amps for comparison purposes, including a Pass Labs XA30.5, the amp-only version of the INT-30A; the big-watt Pass Labs X350.5 Class AB MOSFET amplifier and an unmodified Bryston 14B SSTII bipolar amplifier.

The audition

With plenty of experience listening to Pass Labs amplifiers, the INT-30A’s sonic signature was instantly recognizable. Like the amp-only XA30.5 the integrated brother has a broad sound stage with a lot of space and detail. The all Class-A amp imparts just a tinge of analog warmness, yet the signature is still more on the side of accuracy and speed, rather than the slow and well-warmed signature of a tube amp.

The INT-30A’s amp section is sonically complemented by the passive attenuating preamp that does not impart any sound of its own. It is basically the source gain with the attenuation of its fixed level through the volume control. If done right, passive preamps, long a favorite of purists, allow preamp output to be quite neutral — in that there is no preamp color; you hear the source (and hopefully you like its sound since you bought it) at the desired volume level. This, I believe, explains why the INT-30A sonic signature leans more toward a speedier accuracy than if it had a more traditional active preamp section.

INT-30A Volume Comtrol

Case in point was the playback of Anthony Wilson — Our Gang SACD, one of my favorite jazz recordings. I auditioned the Groovenote SACD on the Esoteric DV-50 and the Oppo BDP-95. The simple direct-to-DSD jazz trio recording features a jazz guitar, organ and drums. It is a warm in the old school tradition, but has plenty of transient energy and a lively sound stage with subtle reverb and room reflections. The INT-30A relayed the recording with the warmth intact, but loses little of the transient energy that I hear in non-Class-A amps — a characteristic that I attribute to the passive preamp.

Multiple tracks from the DMP SACD Steve Davis — Quality of Your Silence yields similar results. This live-to-two track DSD recording has great percussion and accurate tracking of a Steinway piano that is uncannily real through a top-notch amplifier. The INT-30A, again, impressed in its piano tone delivery. The almost three-dimensional piano notes were intact, with just a bit more warmness than my bipolar output amps or the Class-A/B Pass X350.5. Impressive.

With any music I fed through the INT-30A, it did an excellent job with accuracy, yet its Class-A operation imparts a bit extra pureness of the analog sound. A touch of warmth, but the sound is really not colored. Classical music reproduction through the INT-30A was exquisite.

With any music I fed through the INT-30A, it did an excellent job with accuracy, yet its Class A operation imparts a bit extra pureness of the analog sound. A touch of warmth, but the sound is really not colored. Classical music reproduction through the INT-30A was exquisite.

Through the INT-30A, my Mercury Living Presence SACD of Janos Starker’s 1960s sonic masterpiece, Bach Solo Cello Suites was full bodied with all the subtle textures of Mr. Starkers’s bow technique; I could hear all the string harmonics, Starker’s breathing and all the little sounds that a good system brings out. Not one bit of stridency in the overall sound. I love this recording on good audio gear. (Unfortunately, it is no longer available as an SACD, but is available as a PCM download from HD Tracks.)

I also sampled several of the 2L classical music Blu-rays, and they were just as impressive through the INT. String instruments, in particular, were reproduced very accurately with all the speakers. Even the singe-driver Volti Audio Veretta (review upcoming) impressed me, when powered by the Pass. Violin solos, in particular.

On Pop music, the 24-bit Beatles FLAC versions of the 2009 album re-releases were delivered with the verve of the finest audiophile separates. If you have not heard the hi-res versions of the Beatles, you should seek them out. They show how good the 1960s analog tape format and engineering could be. Way more sonic detail and space than the CD versions.

Operationally, the Class-A Pass integrated was faultless. The buttons all engaged with an authoritative click, the beefy aluminum remote operated smoothly, and there were no unusual start up noises or turn-off thumps. It all worked smoothly.

With my balanced and unbalanced sources, they all provided enough gain to drive the speakers to amply loud levels. However, some low-gain sources may not match well with the Pass. If considering this integrated, it pays to choose your components carefully. The Oppo BDP-95, the Esoteric DV-50, and the fixed outputs of Lavry and Benchmark DACs also had no problem with the Pass. Some standalone phono preamps may not have enough level to allow louder levels through the INT.

My only wish for the INT-30A would be an onboard headphone amp. Since the integrated’s concept is to combine features of amp an preamp in one chassis, why not complete the package and add a headphone amp. Just asking, Mr. Pass?

The verdict

Overall, as with every Pass Labs product I have reviewed in the last 15 years, the INT-30A is a superbly engineered hi-fi component. The sum of a Class-A amplifier and passive-attenuating preamp equals a worthy, high-end audiophile amp sound, yet keeps the footprint to just one component. And the INT-30A’s only frill is the preamp output which adds extra flexibility, allowing you to use the passive preamp with another amp, operating a second zone or feeding L and R signals to a pair of subwoofers.

For the serious audiophile who wants great sound and source-switching flexibility in one chassis, the Pass INT-30A is a perfect choice. The Class-A amplifier operation and the passive preamp give it a slightly warm analog signature with a wonderful soundstage, but plenty of transient energy. And it looks like and feels like a million bucks! No scrimping on this amp. Giving it an Everything Audio NetworkStellar Sound Award, a recognition list that includes the Pass XA30.5, the INT-150 and dedicated Pass phono preamps, is a no brainer.